tt system

[Peter Woodford]

guys,

not got my books handy at mo is the maximum Zs on a tt system 200 ohms

 

[pevvers]

If you're NICEIC then it's 100 ohms, if you go by BS7671:2008 then it's 200 ohms.

 

[Sintra]

In theory the maximum Zs permitted by BS 7671 on a TT system with a 30mA RCD is 1667 ohms.

50 / 0.03 = 1666.67

 

[Guest123]

Max Zs is 1667ohms if a 30mA RCD is present (which it should be)

Max Ze/Ra is 200ohms.

 

[pevvers]

Yeah, I suppose they are both good points, but would either of you accept a TT system that had a Ze of 1664 ohms, seeing as the Zs would be unlikely to be above the 1667 ohms that you state?

I know I most certainly wouldn't...

 

[Guest123]

Well no, because as we've all stated the max Ze for a TT is 200ohms or 100ohms if you're an NIC man.

The OP asked about the max Zs on a TT, which if the circuit has a 30mA RCD protecting it is 1667ohms (50V touch voltage). As you rightly say it will never be anywhere near this high and if it were then we would definately do something about it.